Bt crystals, sometimes referred to as insecticidal
crystal proteins (ICP), are protein crystals formed during sporulation
in some Bt strains. Bt produces proteins that aggregate
to form a crystal.

These crystal proteins are toxic to very specific
species of insects yet harmless to humans
and the natural enemies of many crop pests (benenificial insects). There
are more than 150 insects that are known to be susceptible in some way
to Bt.

The crystal proteins bind specifically to certain
receptors in the insect's intestine. Not all insects carry the same receptors
allowing for high species specificity. Humans and other vertabrates do
not have these receptors in their bodies, so the toxin is unable to affect
us.